Secondary batteries have come into widespread use, not just as power supplies for portable devices such as mobile phones, digital cameras and laptop computers, but as power supplies for vehicles and household appliances. From among the different kinds of secondary batteries, lithium ion secondary batteries, which have high-energy density and are lightweight, are energy storage devices that have become essential in daily life.
Secondary batteries are generally classified into spiral-wound type secondary battery and laminated type secondary battery. A battery electrode assembly of a spirally-wound type secondary battery has a structure in which a long positive electrode sheet and a long negative electrode sheet overlap each other with a separator interposed therebetween are spirally wound in a plurality of turns. A battery electrode assembly of a laminated type secondary battery has a structure in which positive electrode sheets and negative electrode sheets are alternately laminated with separators interposed therebetween. In both a spiral-wound type secondary battery and a laminated type secondary battery, a battery electrode assembly and an electrolyte are contained in an exterior container. Patent Documents 1 and 2 each disclose a configuration in which a sheet having high thermal conductivity is wound around a battery electrode assembly.